Mac OS X never really goes to sleep when plugged in

If you leave your power cord plugged in, your Mac never really goes to sleep, it keeps talking on the network to gmail, dropbox etc...

It looks asleep, the light behind the apple goes off, but if you check network traffic on your router you'll find that it keeps chatting to all its friends.

Same thing if it is completely asleep and you plug it in, traffic picks up immediately.